1. Field of the Invention
The present invention relates to a well bore cleaning tool for attachment to a well casing or the like. More particularly, it concerns a well bore cleaning tool for cleaning the walls of a well bore where there is very limited space or distance between these walls and the well casing, often referred to as "close tolerance" conditions.
2. Background of the Invention
In the drilling and completion of an oil and/or gas well, there are three basic types of cementing operations necessary. The first, termed "casing cementing", is done to seal an annular space between the well casing and the bore hole so that fluids in permeable layers may not migrate vertically in the annulus. This procedure, also referred to as "primary" cementing, involves pumping a desired volume of cement slurry down the casing and back up the annular space a required distance toward the surface. "Channeling" or partial by-passing of the cement slurry past the drilling mud media in the well bore often results in primary cement jobs of poor quality, requiring secondary or "squeeze cementing" if the primary cementing of the casing was inadequate in sealing or supporting the casing string. This second type of cementing operation in well drilling and completion requires pumping of a cement slurry under very high pressure through holes which must be perforated through the casing at precise levels. It can reduce the oil or gas producing capability of a well due to filling the permeable section of the reservoir rock with drilling mud and cement. It is also very costly from the standpoint of the additional time and expense of the extra operations required. It has been demonstrated and documented in both laboratory testing and field operations that rotation of the casing, while cementing, is one of the most important factors in obtaining successful primary cement jobs consistently. The number and size of casing strings required for a well depends on its planned depth and the pressures anticipated at the various sub-surface levels in the area. Typically, deeper wells require a number of concentric strings of casing to be run and cemented in progressively smaller hole sizes to line the newly drilled hole interval to contain the ever-increasing pressures which normally occur in deeper rock layers. For example, a bit which is used to start the well at the surface may be 26" in diameter to drill a 26" hole down to the first casing setting level, where a 20" O.D. casing is installed and cemented. The next bit would be 171/2" in diameter to drill that size hole down to the next required level, where a 133/8" O.D. casing is installed and cemented. The next bit would be 121/4" in diameter to drill down to the next critical level, where a 95/8" O.D. casing is installed and cemented. The next bit size would be 81/2" in diameter to drill the hole down to the next casing level, where a 75/8" O.D. casing is run and cemented, usually as a "liner", which is defined as a short casing string which covers only the open hole interval plus a small overlap into the bottom of the previous casing to save the expense of running the entire length of the 75/8" O.D. casing back to the surface. A 61/2" bit is then used to drill that size hole below the 75/8" casing liner down to the next critical level, where a 51/2" O.D. casing could also be run as a "liner". If the objective zone has still not been reached, a 45/8" bit is used to drill below the 51/2" O.D. casing liner in preparation for a well completion in a still smaller casing size. From this example, it becomes evident that the annular space available between the bore hole and the casing or casing liner in deep wells is very limited and a cleaning tool which can be used safely to aid in the successful primary cementation of casing under such limited clearance conditions is greatly needed in the oil and gas industry to improve the quality of well completions while reducing their ultimate cost.
The setting of "cement plugs" is the third type of cementing operation sometimes required in drilling of oil and/or gas wells. It consists of filling up the drilled borehole over a specified vertical interval, usually ranging from a few hundred feet to a thousand feet or more in length, with cement to effect a "cement plug", for the purpose of abandoning a well found to be dry or depleted of oil or gas, or to change the direction of drilling to drill around a section of drill pipe or casing which may have become "stuck" and prevented deeper drilling, or to direct the drilling of the borehole in a different direction to a more favorable subsurface position to find the oil or gas reservoir. In setting "cement plugs", it is desirable to do so with pipe smaller than the drill pipe, since its removal, after the cement slurry has been pumped into place, does less to disrupt the ability of the cement to form a solid barrier in the borehole.
Whether cementing casing or setting "cement plugs", it is desirable for the walls of the well bore to be mechanically cleaned of the uncirculatable mud media, sometimes called "mud-cake", using abrading devices of cable or wire, known as "scratchers", to prevent contamination of the cement slurry by the chemically treated drilling mud and permit better bonding of the cement to the cleaned bore hole.
One common type of well bore cleaner is one which comprises cable and collars, the collars fitting on a pipe casing, and a cable or cables connected between the collars. The collars are securely attached to the casing, such as by welding or using set screws. Once the collars are attached to the pipe casing, the pipe casing is inserted into the well bore and, depending upon the particular arrangement of the collars and cables, the pipe casing is either rotated or reciprocated (or both), during which process the cables frictionally contact the well bore walls and loosen up the mud cakes; at the same time, cement is pumped through the bottom end of the casing upward and displaces the drilling mud and the mud cakes, which are loosened by the well bore cleaners, from the annulus. In some cases, wire is tied or otherwise attached to the cables in order to aid in cleaning the bore walls, as can be seen in U.S. Pat. Nos. 2,868,298 and 2,868,299. The well bore cleaning tools disclosed in these patents comprise, as do all well bore hole cleaning tools comprising cables and collars with which the applicant is aware, either a plurality of cables, or a single cable which is twisted around the casing to form a helix.
The limitation of previous rotating type cleaning devices, known to the applicant as "scratchers", has been that their cleaning fingers or loops are mounted on longitudinal metal strips, usually five feet long. These have certain disadvantages when used in close tolerance annular conditions encountered in deeper wells; their installation without welding requires separate stop devices, the combination of which protrudes from the casing surface too far to be safely used; the protruding devices may contact a ledge and any minor slippage of one stop device will cause the longitudinal base strip to buckle or break off and jeopardize lowering the casing or liner further to the desired setting depth. Additionally, the void space under the longitudinal strip, between it and the casing surface, is difficult, if not possible, to fill with cement, leaving a new area in the annulus for undesirable vertical fluid migration.
The scratcher shown in the applicant's U.S. Pat. No. 4,159,742 cleans by either reciprocation or rotation of the pipe, but its design is for setting cement plugs using smaller diameter pipe (then the drill pipe) where annular tolerances are not close, plus its 360 degree wall abrading design could be detrimental when lowering the casing in a deep close tolerance hole where there is a delicate mud balance between losing mud (to formations) and having one or more of the uncased permeable zones in the well trying to flow.
For the above reasons, the cleaning tools known to the applicant cannot be used as safely or effectively in close tolerance annular conditions as the well bore cleaning tool of the present invention.